Stallion-like Behaviors

I castrated my gelding at 17 months (November 2005). A veterinarian with 35 years of experience showed me the epididymides. My mare had her first heat cycle in March (2006). He mounted and bred her as if he was a stallion. Every time she is in heat, he does this. He does not act "studdy" at any other time, and he is only like this when he's at pasture. If he is being handled near her while she is in heat, he shows no interest.

A 9-month-old colt she is pastured with also attempts. He achieves an erection and mounts, and if he was tall enough, he would breed her. Recently, an 8-year-old gelding was placed in the pasture, and he, too, mounts her. He has never done this to any other mare. Are the others just copying my gelding? What caused my gelding to do this in the first place?       Kris Bark, via e-mail

Your situation and questions raise several points concerning stallion-like behavior in geldings that can be very frustrating for horse owners. This has to be on the top 10 list for frequent questions about male behavior.

I'll answer by explaining why geldings and juvenile males "play mount" mares at pasture. And for geldings who appear fairly serious in their sexual behavior, we should also discuss other possible explanations.

But first, let me comment on your mention of the veterinarian showing you the epididymides when he castrated your colt. Perhaps you mentioned that because of the issue of leaving some or all of the epididymides behind to create what is known as a "proud-cut" gelding.

Before research showed differently, it was believed that the epididymis produced male hormones and that leaving some or all of the epididymides at the time of castration would leave the gelding with a small amount of male hormones. The expected advantage would be that the proud-cut gelding would perhaps retain a bit more of the showiness, athletic drive, and work ethic of a stallion, without the challenge of the unwanted or unmanageable sexual and aggressive behavior of an intact stallion.

It has been decades since science has shown that the hormones that could affect stallion-like behavior actually originate in the testicle. The hormones arrive from the testicle either fully produced or in a near-complete form. They undergo conversion into the final active form in the epididymides. If one or both of the epididymides are left behind when the testicles are removed, the steroid hormones and raw materials for conversion that came from the testicles dissipate. Perhaps there is something from the epididymides that has yet to be discovered that affects behavior. Even the question of whether behavior is really different in proud-cut stallions has not been studied in organized research.

So in the case of stallion-like behavior in a gelding, the first important question is: Both testicles were fully removed at the time of castration? A fairly common scenario is that one of the testicles is not in the scrotum at the time of castration, so it is not removed. It can be out of easy reach high in the inguinal area, or it can be all the way up in the abdomen. It would require some diagnostics to locate a remaining testicle and more complicated surgery to remove it, and so it is just left in many cases. The usual plan is to wait and see how the "cryptorchid" gelding does.

A less-common finding is that a small piece of testicle is inadvertently left. This "remnant" can still produce enough male hormones to drive sexual behavior. For your gelding, neither of these are likely the case, since you owned the horse at the time and attended the castration.

Another reason complete castrates show stallion-like behavior is that no matter how old or experienced colts are when castrated, there is a wide range of retained stallion-type sexual and aggressive behavior.

Some geldings mellow out to having almost no recognizable male-type behaviors, while others are indistinguishable from intact stallions. This is not just the overt interest and sexual response to females, but all of the male-typical behavior. For example, some confirmed geldings still always defecate in the same place, forming stud piles like stallions.

Your comments on your gelding showing sexual behavior only at pasture is also a common observation of gelding owners. There is more to stallion behavior than just testicular hormones. Many geldings--no matter how young or old and whether or not they had sexual interactions or breeding experience before castration--retain stallion-like behavior. Their sexual and aggressive behavior is much more muted usually, and it is more easily controlled with usual training and handling methods than it is for intact stallions. But at pasture, or uninhibited by people or by the presence of more dominant males, the stallion-like herding, teasing, and mounting seem to blossom, and the behaviors are, of course, very difficult to control.

A common question is whether the administration of female hormones such as progesterone, that appear to somewhat subdue stallion-like behavior, is useful for stallions or geldings at pasture. Experience has been that they usually are judged as not at all effective for pasture situations, even though they might seem helpful in quieting a stallion or gelding for work or show situations.

So why are the 8-year-old gelding you describe, who has not been known to show residual stallion-like behavior in the past, and the 9-month-old colt now mounting your mare?

Well, there are a couple possible reasons. Geldings, like some stallions--especially those with low libido--can be more or less not interested in most mares, but every once in a while they might find one particularly stimulating. Perhaps this mare is just one of those particularly stimulating mares.

Another possible factor is that sexual interaction of any pair is typically sexually arousing to herdmates. You see this effect in natural breeding situations, both with youngsters and with bachelor stallions.

When teasing and breeding occurs among a harem stallion and mare, or between a bachelor stallion and a young female, it appears to excite activity in other males, both mature or immature, whether in the play form or in inter-male mounting among the bachelors. We often use this natural tendency to attempt to arouse slow-starting novice breeding stallions in domestic breeding conditions. It has been called the voyeur effect. It doesn't always work, but we sometimes try.

For the 9-month-old colt you describe, his mounting is likely play behavior, even with an erection.

About the Author

Sue McDonnell, PhD, Certified AAB

Sue M. McDonnell, PhD, is a certified applied animal behaviorist and the founding head of the equine behavior program at the University of Pennsylvania's School of Veterinary Medicine. She is also the author of numerous books and articles about horse behavior and management.

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